\section{Chondrites}
\label{appendix:chondrites}

Chondrites are stony meteorites that have round grains "chondrules" in them.
See Fig. \ref{fig:chondrite} for example.

\begin{figure}[h!]
\centering
\includegraphics[width=\figuresize\textwidth]{figures/chondrite.pdf}
\caption{
A slice from the 4.5-billion-year-old Allende meteorite. This rock was 
formed along with the solar system. Figure from wikipedia. 
(\url{http://en.wikipedia.org/wiki/File:Allende_meteorite.jpg}) 
}
\label{fig:chondrite}
\end{figure}

CI chondrites, a small group of meteorites, have the most primitive 
compositions, meaning that physical and chemical processes that formed the
meteorites did not significantly change their elemental compositions. This
conclusion comes from plots like Fig. \ref{fig:ci}, which shows the 
abundances of elements in CI chondrites versus the abundance in solar 
photosphere. Elements on the solid line in Fig. \ref{fig:ci} are present
in the same abundance in the Sun and in CI chondrites. Except for those
volatile elements that exist in the gas phase at room temperature and
elements that were burned out of the Sun by the first stages of nuclear burning
(like Li), essentially all elements are with the same relative abundance
in the Sun and the CI chondrites. Since most of the meteorites were from
the collisions of asteroids, which are generally small and have not experienced
significant fractionation like the Earth, this correlation clearly indicates 
that the mixing during the early stages of the solar system was quite 
efficient. 

\begin{figure}[h!]
\centering
\includegraphics[width=\figuresize\textwidth]{figures/ci.pdf}
\caption{
Elemental abundances in CI chondrites compared to abundances in the solar 
photosphere. For comparison
purposes, the abundance of Si is set at $10^6$ Si. The bulk compositions of 
the CI chondrites and the solar photosphere show an amazing correlation, 
with a few key exceptions. Combined, these data sets provide our best 
estimate for bulk solar elemental abundances. H and the noble gases are 
not shown. Figure and caption from Lauretta, A Cosmochemical View of the Solar
System, Elements, Vol. 7, PP. 11-16.
}
\label{fig:ci}
\end{figure}
